Elderly populations display a greater susceptibility to infection, anemia and other hematological diseases probably due to a decrease in hematopoietic function. Studies on human populations of different ages have indicated an altered hematopoietic mechanism occurs in the elderly, however, little detailed information on the causes has been conclusively obtained. Animal model studies have provided information on augment evidence that the hematopoietic ability is lower in aged groups than in the young, but systematic approaches to determine the source of the defect have been lacking. In this proposal the specific effect of aging on myelopoiesis in a rat model system will be studied by investigating the two major factors that regulate myelopoiesis: the availability of myeloid progenitor cells (CFU-GM) and production of the myeloid maturation hormone; colony-stimulating factor (CSF). Alveolar macrophage and spleen lymphocyte cells, two of the most active CSF producing cell systems, will be used to study the responsiveness of the cells to mitogen stimulation. Contradicting results have been reported in regard to the CSF level in different aged populations of humans and animals due to lack of proper controls. The results obtained here should provide some insight into age related alterations in myelopoiesis and allow further biochemical characterization of the system.